1. Field of the Invention
The present invention relates generally to techniques for etching the surfaces of semiconductor substrates, primarily silicon. In particular, the present invention relates to the fabrication of nonreflective silicon solar cells.
2. Description of the Prior Art
The direct conversion of sunlight into electrical power promises to provide energy which is both abundant and inexpensive. Among the leading techniques for achieving the conversion is the use of photovoltaic power systems which use semiconductor solar cells. The success of such systems depends on the availability of solar cells which are economical and efficient. Significant advantages have been made recently although the cost relative to fossil fuels is still prohibitively high.
The standard materials for semiconductor solar cells are silicon and cadmium sulfide, although a number of other materials, such as gallium arsenide, indium phosphide, amorphous silicon and other materials or combinations thereby, may be useful. Because silicon is the most highly-developed semiconductor technology, it is the most promising material for solar cells. It will be necessary to substantially increase the efficiency of the materials in converting sunlight into electrical energy. The principal efforts for reducing the cost of current silicon designs include automation of device processing, reducing the cost of materials and development of low-cost materials such as polycrystalline and metallurgical silicon.
Another avenue for achieving economy is to reduce the reflectivity of the silicon surface. The principal techniques for doing so involve depositing a non-reflective coating atop the silicon surface or to create a textured upper surface which collects a large percentage of reflected light and minimizes the distance which photogenerated carriers must diffuse before collection. Such non-reflective structures may yield efficiencies as high as 20 percent for monocrystalline silicon, although conversion efficiencies in production quantities are usually around 12 percent or so.
Possibly the best known of the silicon solar cells produced with a serrated or textured surface is the COMSAT non-reflecting (CNR) cell. In the CNR cell, pyramidal surfaces are produced on &lt;100&gt; oriented wafers by preferential etching techniques, one of which is a hydrazine-hydrate etch. The cell achieves reduced reflectivity, less dependency on antireflection coatings and increased collection efficiency due to carrier generation closer to the junction. The reflectivity of bare silicon is reduced from 35 to 45 percent for flat surfaces to around 20 percent for the textured surface, and the addition of an antireflection coating reduces the overall reflection to a few percent. The CNR cell has been described in greater detail in the text by Harold J. Hovel entitled "Semiconductors and Semimetals" , Vol. 2, Solar cells, Academic Press, 1975, pp. 225-227.
As noted, the production of the CNR cells involves wet etching. As is well known in semiconductor art generally, wet etching is at times difficult to control, particularly because of contaminants which may be contained in the etching fluid or the precise duration of the etching step needed to produce acceptable results. Another problem with the wet etching process to produce the textured surfaces is that it is effective only on single crystalline silicon. This is the most expensive type of silicon; and it would be desirable to produce textured surfaces on so-called "solar grade" silicon, which is considerably cheaper and adaptable to mass production.
Another well-known technique for etching silicon involves dry RF plasma etching using halocarbon mixtures. Other dry etching techniques involve physical sputtering such as DC or RF sputter etching and ion milling procedures.
Plasma etching involves the chemical combination of the surface of the material to be etched with a gaseous species produced in a discharge. It is distinct from physical sputtering which involves ion bombardment only.
Such techniques have been used not only to etch silicon and other materials but have also been used to clean their surfaces of contaminants and also to remove materials, such as silicon dioxide, and photoresist from its surface. We have found, however, that neither sputter etching, ion milling, nor plasma etching techniques will yield a non-reflective cell having the characteristics of the CNR cell. In particular, a plasma tool where the RF power is coupled outside the chamber is not effective; the silicon is merely etched.